Reflective direct read-after-write metal laser recording media have been used in the past for optical data storage and recording. Typically a thin metal film of tellurium, bismuth or selenium is vacuum sputtered or evaporated on a substrate thereby forming a laser recording medium in which holes representing data are ablated in the thin low-melting temperature metal surface by laser energy. Such data may be read by either transmitted or reflected light.
U.S. Pat. No. 4,269,917 (Bouldin and Drexler) a reflective laser recording and data storage medium is described having an electrically non-conductive surface layer of reflective silver particles in a low melting temperature colloid matrix. Such a dispersion of silver particles in a colloid such as gelatin results in a more sensitive laser recording medium than thin silver films since there is less loss of recording energy because of the lower thermal conductivity on the electrically non-conductive silver containing surface. Laser recording sensitivity may be further improved by the presence of an energy absorbing underlayer as taught by U.S. Pat. No. 4,312,938 (Drexler and Bouldin) which acts to concentrate depthwise the recording beam energy in the recording spot on the reflective surface thereby creating a melting of the underlying colloid causing a depression or pit in that area. Such depressions representing data would have decreased reflectivity compared to the upper unpitted reflective surface.
Laser recording media of the type using silver as the reflective metal are subject to corrosion if the atmosphere to which such media are exposed contains sulfide or chloride. Tarnishing of the silver to gray-black silver sulfide reduces the contrast ratio of the recorded data-containing low reflective areas to the reflective surface with a concomitant loss of data reading sensitivity. This decreaseed sensitivity requires higher reading laser power than normal.
Problems of noise may arise when the corrosion-caused silver salts are formed on the surface. Phase shifts and scattering of reflected light, distorted by surface dirt, cause apparent amplitude changes and reading errors.
The protection of the silver from corrosion must be by a means which will not result in a decrease in the reflectivity of the recording surface which would necessitate an increase in the laser power required to read the data after recording because of the diminished contrast ratio. A layer of smaller diameter metal grains on the surface, as taught by the anti-reflective layer of Lou et al., U.S. Pat. No. 3,889,272, would lessen the amount of laser energy required during recording by lessening the amount of reflection losses but would decrease the reflectivity of the medium necessitating higher laser energy for playback during reflective reading of the data. This might present problems for inadvertently recording depressions or pits with the read beam during readback of the data on the recording medium.
Any protective coating must not cause the recording medium to lose the advantage of the higher recording sensitivity. Non-conductivity of the recording surface in U.S. Pat. No. 4,269,917 allows lower laser recording energies than a conductive metal layer type of media and such nonconductivity must be preserved by any coating.
An object of the invention is to devise a protective coating for a reflective silver recording surface which would protect the reflective surface from atmospheric corrosion and extend the life of the product without causing loss of reflectivity or interfering significantly with electrical non-conductivity of the surface reflective silver particles.
Another object of the invention is to devise a protective coating which will maintain the reflectivity of the recorded surface at a relatively constant level and require less laser power for recording data pits.